1. Field of Invention
The invention relates to thermophotovoltaic power generators for converting excess heat from industrial processes to electricity. In embodiments, the present invention relates to the use of such generators in various aspects of a silicon crystal growing process or plant, for harnessing excess heat and converting the heat into useable energy.
2. Description of Related Art
Photovoltaic cells, more commonly known as solar cells, made of silicon or GaAs have been used to generate electricity from visible light energy. Solar cells have been used to power handheld games and calculators as well as to generate electricity in homes and businesses where solar panels have been incorporated into the roof or other suitable structures. However, solar cells typically generate only 0.01 watts/cm2 in direct sunlight. Thus, in addition to the quality of the light (e.g., direct sunlight or sunlight on an overcast day) the electrical output of these cells has been limited by the surface area exposed to available light sources.
Between 75 to 90% of the available energy is in the infrared (IR) spectrum, not the spectrum of visible light. Unfortunately, the spectrum of IR radiation is beyond the band gap of silicon and GaAs cells, although it is within the band gap of GaSb cells. These GaSb, or thermophotovoltaic (xe2x80x9cTPVxe2x80x9d) cells, respond to infrared radiation rather than visible light energy from light sources, such as the sun. Thermophotovoltaic generators for converting fuel energy into electrical energy have been described. See U.S. Pat. Nos. 5,512,109, 5,616,186, 5,651,838 and 5,942,047 to Fraas et al.; U.S. Pat. Nos. 5,551,992 and 6,037,536 to Fraas; U.S. Pat. No. 5,865,906 to Ferguson et al.; and U.S. Pat. No. 6,218,607 to Mulligan et al. The generators described are powered by hydrocarbon fuels such as natural gas, propane, butane, or kerosene. These generators primarily generate lower power outputs suitable for purposes such as residential or space heating.
GaSb TPV cells have been shown to be capable of generating electricity reliably and economically. The integration of TPV generators into portable or residential heaters reduces energy costs and provides a measure of independence from outside power generation systems that have been prone to interruption or failure.
Many industrial processes generate intense amounts of heat that, more often than not, is simply dissipated into the environment. The tremendous heat output of these processes can instead be harnessed to generate electricity. The heat generated by processes performed in a wide variety of apparatus such as, for example, electric, natural gas, oil and other furnaces for melting ore, growing crystals, making glass, annealing materials, containers for any exothermic chemical processes, or various heat exchanger systems can thus be advantageouly be harnessed to produce energy that can be used elsewhere in a plant or that can be fed back into the apparatus.
In the past, a common practice has been to utilize appropriately placed heat exchangers around various process units, to harness some of the excess heat energy for use elsewhere in the production process. Thus, for example, heat exchangers have been widely used around exothermic reaction vessels to use the generated heat energy to produce steam or heated water, which can be used in other process units. This has resulted in cost savings and efficiency gains, since other energy sources are not required to heat the process water. The present inventors have discovered that the excess heat produced by these various processes can thus alternatively or additionally be harnessed by thermophotovoltaic cells, to provide a source of recovered energy that likewise can be returned to the individual process unit, or recirculated into the power grid for use elsewhere. Generally, the above described processes must be performed either under strict temperature controls or on a scale such that thermal inertia permits greater uniformity of temperature from a given heat source. The present inventors have discovered that heat output of these processes can also yield higher power outputs from TPV generators.
When GaSb TPV cells are exposed to an IR source, the cells absorb IR radiation having a wavelength of 1.7 xcexcm and convert the heat energy to electrical energy. Optimal efficiency with the TPV cells is achieved when the temperature of the IR radiation source tube is nearly constant. Thus, industrial processes would be able provide IR radiation at amounts and wavelengths that are more stable than in smaller portable or residential sources.